1. Field of the Invention
The present invention relates to a control circuit for controlling a gear-shift unit for shifting a mechanical-type automatic transmission or a clutch control unit for applying/releasing a clutch interposed between an engine and the mechanical-type automatic transmission, and also to an abnormality determining device for the control circuit.
2. Description of the Related Art
As a transmission for an automobile, there has been a mechanical-type automatic transmission, which is provided with a speed-changing gear mechanism and a clutch mechanism similar to those of a manual transmission, an actuator (or a gear-shift unit) for driving the speed-changing gear mechanism and an actuator (or a clutch control unit) for driving the clutch mechanism, and an electronic control unit (ECU) for controlling those individual actuators. This mechanical-type automatic transmission has been centrally applied to a large-sized vehicle such as a truck or a bus.
Here, in a vehicle equipped with an air pump, the gear-shift unit can be constructed with actuators using pneumatic pressure. In a vehicle not equipped with an air pump, however, it is more advantageous in terms of the mounting space and cost by utilizing electric actuators rather than providing a new air pump.
In the case of the clutch control unit, it is also more advantageous to utilize electric actuators which in combination with an electric pump for producing a fluid pressure or an electric motor.
In the case of the mechanical-type automatic transmission having such electric actuators, a structure, as shown in FIG. 7, can be generally considered. More specifically, as shown in FIG. 7, a driver circuit 33 for controlling operations of a gear-shift unit 31 and a driver circuit 23 for controlling operations of a clutch control unit 21 are integrated, and that the driver circuits 33 and 23 are connected via signal lines 81 and 82 with an ECU 41 for controlling those actuators 31 and 21.
On the other hand, electric current for driving the gear-shift unit 31 or the clutch control unit 21 has to be fed from a battery 70 through the driver circuits 33 and 23 to the gear-shift unit 31 or the clutch control unit 21. It is, therefore, necessary to provide a power feeding harness (or a power feed line) 73 from the battery 70 mounted on the chassis side to the driver circuits 33 and 23 to provide power feeding harnesses (or power feed lines) 74 and 75 from the driver circuits 33 and 23 to the gear-shift unit 31 and the clutch control unit 21 mounted on the chassis side.
The driver circuits 33 and 23 are arranged apart from the ECU 41, as described above, for the following reasons.
The actuators, such as the gear-shift unit 31 and the clutch control unit 21, are disposed in the vicinity of an engine, which has a high temperature and a serious vibration. This is not a suitable environment for the ECU 41. Therefore, the ECU 41 is arranged apart from the gear-shift unit 31 and the clutch control unit 21.
Moreover, the actuators, such as the gear-shift unit 31 and the clutch control unit 21, have to be fed with a large current so that a large capacity electric wires for the large current have to be used as the harnesses 73, 74, and 75. To suppress increase in cost and weight, therefore, it is effective to reduce the lengths of the harnesses 73, 74, and 75. The lengths of these harnesses 73, 74, and 75 can be reduced, if the driver circuits 33 and 23 are arranged apart from the ECU 41 and close to the gear-shift unit 31, the clutch control unit 21, and the battery 70.
However, in a system where an electric actuator utilizing the electric motor or the like, or a solenoid valve is adopted in the gear-shift unit or the clutch control unit, the following problems occur if the driver circuits 33 and 23 for driving the electric motor or the solenoid valve are separated from the ECU 41 and arranged near the battery 70 so that the drive power may be taken directly from the battery.
In case any abnormality occurs in the system, a malfunction may be caused despite no drive signal is provided from the ECU, and the vehicle may exhibit unexpected behaviors.
If a minute current flows in the drive signal line when the engine key is OFF, the battery may be exhausted by the malfunction of or the power consumption by the actuator.